1. Field of the Invention
The present invention relates to a camera and, more particularly, to a camera which uses, as a light receiving means, a signal integrating and accumulating type image sensor for receiving light from a photographing optical system prior to the photographing operation and for producing an output which may be utilized for a predetermined purpose. The integrating time of the image sensor is controlled on the basis of the output thereof.
2. Description of the Prior Art
Various focus detecting systems for cameras has been proposed and many of them are already available on the market.
Although various types are known for these focus detecting systems, the focus detecting systems of so-called TTL (Through The Lens) type are particularly suitable for single reflux cameras. In the TTL type the light from a photographing lens is received by a light-receiving element and the focusing state of the photographing lens is detected on the basis of an output signal from the light-receiving element.
The same assignee of the present invention has already proposed several focus detecting systems of the TTL type wherein a signal integrating and accumulating type image sensor such as a photodiode array, a CCD (Charge Coupled Device) or a BBD (Bucket Brigade Device) is used as the light-receiving means. The image formed by the imaging optical system is scanned by the image sensor, and sharpness of the image formed on a predetermined imaging plane is detected on the basis of the scanning signals obtained by the scanning operation to thereby detect the focusing state of the imaging optical system.
When the signal integrating and accumulating type image sensor is used as the light-receiving means in place of the conventional photoelectric transducer elements, the incident optical energy is converted into electric signals (e.g., charge) which are then integrated and accumulated for a desired period of time and are thereafter output as time-serial signals. Therefore, the sensor elements can be made extremely small, so that signals of high precision may be obtained as the scanning signals. Furthermore, the signals may be easily optimized by controlling the signal integrating time, without a mechanical means for controlling the amount of incident light such as a diaphragm. Because of such advantages, the system ensures high performance in focus detection.
The signal integrating time may be controlled in various manners, such as, e.g., by an output from a separate photometer. In the proposals described above, which are made by the same assignee of the subject application, a more efficient method is adopted according to which the luminance information of an object to be photographed is obtained from the output of the image sensor, and the signal integrating time in the next signal integrating operation is controlled on the basis of this luminance information.
When a focus detecting system of this type is incorporated into a single reflux camera, the problems described below occur. In general, so light is incident on the image sensor during photographing (film exposure). Therefore, if the signal integrating time is being controlled during this time, the signal integrating time of the image sensor is set at the maximum extent of the adjustable period of time. Then, it takes a certain amount of time before the signal integrating time is restored to the optimum value for the next photographing operation. This results in a low response of the system or unstable operation. In particular, when continuous photography is to be performed, the shutter chances may be missed.